1. Field of the Invention
The present invention relates to a brush mounting structure for an ercoder and a focal length detection mechanism, in a zoom lens barrel.
2. Description of the Related Art
In a zoom lens camera, a focal length is detected by obtaining focal length data, via the relative sliding movement between a code plate having a predetermined circuit-path (pattern) and a brush, provided in a lens barrel. The code plate is in the form of a thin sheet which is received into, and adhered to, a shallow recess formed on a cylindrical peripheral surface of a thin lens barrel member, so that the code plate is substantially flush with the cylindrical surface of the lens barrel member. The brush is secured to another lens barrel member usually by means of screws and projects from the peripheral surface of the lens barrel member so as to come into contact with the code plate. Therefore, the mounting position of the brush is restricted; and hence, in order to provide such a brush, depending on how it is installed, the zoom lens barrel has to be enlarged, possibly reducing the strength of the lens barrel. Furthermore, upon installing such a brush, depending on how it is installed, there is an increased danger of being unable to completely block out light within the lens barrel. Namely, it is difficult to incorporate the brush in the lens barrel without increasing the size of the lens barrel and while providing sufficient strength of the lens barrel and sufficient light interception.
Moreover, in a known encoder as mentioned above, the brush defines an electrode member which is electrically connected to a focal length detection circuit of a camera body through a conductor, such as a flexible printed circuit board (FPC board). However, in this arrangement, since the number of terminals is increased, the brush is wide and large. To facilitate the electrical connection using the FPC board, the brush must be provided on the camera body or a camera component, which is guided to move linearly with respect to the camera body. Therefore, the mounting position of the brush is restricted.
In a known focal length detection mechanism of a zoom lens camera, the code plate and the brush are relatively moved in a direction parallel with the optical axis. This known focal length detection mechanism has some drawbacks. Namely, in recent zoom compact cameras, the (multi-feed type or multi-stage extension type) zoom lens barrel is usually composed of telescopic multi-stage short barrel elements which are telescopically connected in their axial direction (parallel with the optical axis) to reduce the size of the camera when the lens barrel is in a retracted position and to increase the focal length. The axial length of the code plate is restricted cue to the short axial length of the barrel elements. The code plate is provided with code circuit-paths which provide a predetermined number of split stages corresponding to the number of the split feed stages within the contact area with the brush. If the code plate is short in the axial direction, it is difficult to obtain a sufficient number of the split stages on the code plate, or the code circuit-paths must be designed finely. If no required number of split stages can be obtained, the zooming operation cannot be controlled. An increased fineness of the code circuit-paths leads to a decrease in the detection accuracy or an increase in the manufacturing cost.